The present invention is related to a control circuit for controlling operation of a three phase motor. In particular, the present invention is a motor controller which limits power supply current draw as a hard drive spindle motor is spun up from a stop condition to its normal operating speed.
Spindle motors used in hard disc drives of computers are typically three phase motors having three terminals. When a computer is first started, the hard drive spindle motor is typically spun up from a stopped condition by a motor controller circuit using classic six state commutation, with peak current limiting and a relatively high motor current. A typical spin up of the spindle motor is from zero RPM to 15,000 RPM in about 5 seconds. To spin up the motor as quickly as possible, the start current is set as high as the hard drive power supply can deliver. In some systems, when the spindle motor approaches its normal operating speed (e.g. between about 95 to 100 percent of normal speed) control of the spindle motor is transferred to a second motor controller circuit which provides sinusoidal drive to the three terminals using pulse width modulation.
Peak current limiting during spin up operates by allowing the current through the motor to ramp up to a programmed maximum as one terminal is tied to the operating high voltage and another terminal is tied through a sense resistor to the operating low voltage. While the terminals are tied or connected in this fashion, the motor current ramps up through the motor inductance and resistance between the two terminals. The motor current is measured by measuring the voltage across the sense resistor. When the current through the motor reaches the peak current, the two terminals (which had been connected between a high voltage and a low voltage) are connected to the same voltage. This allows the motor current to decay from the peak current value. Since the motor coil acts an inductor, the motor current does not drop to zero instantaneously. Instead, the current decays at an approximately linear rate. Typically, the off time during which both terminals are connected to the same voltage is not long enough for the motor current to decay completely to zero. Usually the motor current decays by about 10 to 25 percent. When the two motor terminals are switched back to the state where one terminal is at high voltage and the other is at low voltage, the motor current is already at 75 to 90 percent of the peak current, and ramps up from that point until the peak current is again reached. Each time that peak current is reached, two terminals are connected to the same voltage and motor current is allowed to decay.
During the spin up phase of operation, the current flowing through motor from the high operating voltage to the low operating voltage is seen across the sense resistor as a series of short, nearly rectangular pulses. The current is only drawn from the power supply (or a supply bypass capacitor in parallel with the power supply) when the two terminals are in the state where they are connected to different voltages. When the two terminals are connected to different voltages, the current flow is from the power supply into the high voltage terminal, through the motor, and out the low voltage terminal of the motor and through the sense resistor to ground. When the two terminals are both connected to the high operating voltage, the current flow is from the power supply node into the high voltage terminal, through the motor, and back to the power supply node. Therefore, no motor current is drawn from the power supply or bypass capacitor. Current does flow from the power supply into the bypass capacitor during this time. Thus, the current draw is represented by the voltage pulses across the sense resistor. The supply bypass capacitor tends to filter these narrow supply current pulses, which reduces the peak load current seen by the power supply. An average motor current is set by the alternation between the two states in which a pair of motor terminals are connected to high and low voltages or to the same voltage.
With a low resistance motor, the current off time may be significantly greater than the on time. This results in an average current which is significantly less than xc2xd of the peak current. At commutation (which occurs every sixty degrees of rotation) the motor drive circuit switches motor phases so that a different pair of terminals of the three motor terminals are used for connection to the operating high voltage and the operating low voltage. A much wider current pulse occurs immediately following commutation, which increases the effective peak load current. This wider pulse is due to the fact that the motor current must flow through an inductor that previously did not have current flowing through it (i.e. the motor winding connected to the terminal which previously was not connected to either the high or the low voltage). This requires the current to ramp up from zero to the peak current, instead of from about 75 to 90 percent of peak to the peak current. The result is a much longer current ramp time immediately following commutation. This results in a larger current draw from the power supply. In the past, the problem of larger current draw resulting from the longer current ramp time following commutation was addressed by using a power supply having a larger current capacity or by using a larger bypass capacitor. Both approaches result in a more expensive spindle motor system.
A motor controller circuit for a three phase electric motor uses six state commutation. Motor current is controlled by using peak current detection. A peak current target is compared to sensed motor current each time that one motor terminal is tied to an operating high voltage and another terminal is tied to an operating low voltage. When the sensed motor current reaches the peak current target, the two motor terminals are connected to the same voltage, which allows motor current decay. The present invention features xe2x80x9csoft commutationxe2x80x9d in which the peak current target is reduced immediately following each change of commutation state. Subsequently, the peak current target is increased to a higher level for the remaining portion of the commutation state. With the present invention, the effective power supply current spike at commutation is reduced.